Food casings, in particular sausage casings, are frequently offered in a form in which in each case approximately 15 to 100 m of the casing are telescoped together to form 20 to 120 cm long “sticks” (also “strands” or “slugs”). The shirring of artificial skins has long been known and is also described (see G. Effenberger: Wursthüllen-Kunstdarm [Sausage casings, artificial skin], H. Holtzmann Verlag GmbH & Co KG, Bad Wörishofen, 2nd edition [1991] pp. 58 to 60). It is performed on what are termed shirring machines. Before shirring, the casing is laid flat and rolled up. It is taken off from the roll, inflated and pushed onto the shirring mandrel. The outer diameter of the shirring mandrel determines the inner passage width of the stick to be produced. Arranged around the shirring mandrel is a driven shirring tool which grips the casing on the outer surface, transports it axially on the shirring mandrel, and in the course of this plays it into uniform pleats. The shirring tools can have different constructions. Those which are known are, e.g., arrangements of shirring wheels which on the outside can be shaped to be toothed or else smooth, in addition also shirring belts and spiral shirring elements. The friction on the shirring mandrel and shirring tool is a high thermal and mechanical stress for the casing. Immediately before or during the shirring, it is therefore customarily wetted with a lubricant (oil, water or corresponding emulsions) on the inside, from the outside or from both sides. This prevents, inter alia, cracks or other damage occurring at the pleats. Shirring damage can also occur if the degree of compaction (compression density=casing length per stick) is selected to be too high. As soon as the desired length of casing material has passed through the shirring tool, the casing is cut off at the feed and the resultant stick is pushed down from the shirring mandrel.
The resultant stick should be as dimensionally stable, self-supporting and mechanically robust as possible so that it withstands transport, packaging and further handling at the processor without problems. During stuffing with sausage mix, the stick is again “deshirred” in a controlled manner. For this, frequently a plurality of sticks are placed in a storage container from which individual specimens are automatically withdrawn one after the other and pushed onto the stuffing horn of the stuffing machine. It is indispensable here that the stick does not break and retains its preset shape unchanged. Otherwise, faults occur in the automatic sequence of the stuffing operation, which must be alleviated laboriously by hand.
The ability of casings to form intrinsically stable sticks is, depending on the material used, very variable. Currently, sausage casings based on the following materials are predominantly used:                regenerated cellulose        collagen        thermoplastics, of which, in particular, PVDC, polyamide, the latter also in the form of multilayer structures having additional layers of polyolefins and other plastics.        
Solely casings made of regenerated cellulose may be processed (within limits) to form intrinsically stable sticks. The advantage of cellulose here is a certain tendency to form “standing pleats” (like paper). A condition is a defined humidity of the tube to be shirred in order to achieve a suitable ratio of stiffness and suppleness. Frequently, casings made of regenerated cellulose, before shirring, are provided on the inside with an impregnation or solution coating. These treatments serve, primarily, to set in a targeted manner the adhesion of the casing to the sausage surface and to enable easy later detachment of the casing. A multiplicity of impregnating or coating agents have already been described, including those which additionally cause a certain “bonding” of the later shirring pleats to one another and as a result additionally stabilize the stick.
The known impregnating or coating formulas are mostly based on water-soluble and/or water-emulsifiable substances and are introduced into the casing as aqueous preparation. The high swelling capacity of the cellulose is utilized here; as a result, (in the case of not too high inherent moisture of the casing), the water fraction of the introduced preparation is immediately absorbed by the cellulose, and the constituents, after a short time, form a dry coating on the surface. In the case of “bonding” constituents, the shirring is carried out immediately after the coating, so that the solidification of the substances and the bonding of the surfaces first takes place on the finished sticks.
Examples of such “bonding” coating formulas for cellulose casings are:                water-soluble cellulose ethers in combination with an oil or an alkylene oxide adduct of a fatty acid partial ester (U.S. Pat. No. 3,898,348),        water-soluble cellulose ethers in combination with a fatty acid partial ester of sorbitan or mannitol and a water-soluble polyalkylene ether (U.S. Pat. No. 4,137,947),        mixtures of anionic and nonionic water-soluble cellulose ethers and lubricant (EP-A 0 180 207),        lecithin in combination with at least one of the substances alginate, chitosan, casein (U.S. Pat. No. 5,358,784),        mixtures of carboxymethylcellulose (CMC) and polyethylene glycol, if appropriate combined with polysorbate, sorbitan oleate, methylcellulose (MC), lecithin, or mineral oil (U.S. Pat. No. 6,086,929).        
In contrast to the casings based on regenerated cellulose, casings made of plastic are generally considered unsuitable for producing intrinsically stable sticks. Shirred plastic casings, after they are removed from the shirring mandrel, are immediately covered with a tubular net or with a film. Otherwise, there is an unwanted expansion of the sticks in the longitudinal direction (decompacting), followed by opening of individual pleats up to complete loss of shape. This net or film covering must be removed manually by the processor as soon as the stick has been pushed onto the stuffing horn of the stuffing machine. The working step of removal is unwanted, in particular, because it cannot be reliably automated and to this extent the stuffing process overall cannot be carried out in a fully automated manner.
In addition, it is known to shirr plastic casings on a sleeve and to set the shirred material using limit disks (Effenberger, loc. cit., p. 58). This is laborious and therefore disadvantageous.
The possible methods for stabilizing sticks discussed above for casings made of regenerated cellulose are not applicable with customary plastic casings. Firstly, the plastics suitable for casings (polyamide, PVDC) have distinctly elastic recovery properties; a pleat embossed into a film “opens” readily, and the film takes on approximately again its planar shape. Secondly, plastics, compared with cellulose, have virtually no swelling capacity and only low water vapor permeation. An impregnation or coating applied to the interior would remain moist over a long period and not cause the desired “bonding” of the shirred pleats after the shirring process.
The advantages of casings made from regenerated cellulose are, unfortunately, counteracted by their complex and environmentally-polluting technical production. They are generally produced by the viscose method in which the cellulose is first converted into cellulose xanthogenate using sodium hydroxide solution and carbon disulfide (CS2). The resultant viscose solution must first mature for several days before it is passed to the skin spinning machines. These machines essentially comprise a spinning die, precipitation baths, wash and preparation baths and also drying stations. In the precipitation baths the cellulose xanthogenate is regenerated to cellulose. Byproducts produced are hydrogen sulfide (H2S) and sodium sulfate. These substances must be continuously collected and disposed of.
Casings made of customary thermoplastics are, in contrast, technically simple and inexpensive to produce. The customary plastics such as polyamide and polyethylene can be obtained ready-to-process from the manufacturers, and converted directly to casings by extrusion and tubular film blowing or tubular film stretching methods.